I don't think Natsu is a true dual element slayer. He still relies on fire as his primary element and has not displayed any pure/advanced lightning moves. The lightning element is more of an enhancement of Natsu's flame combos imho.

Ever since they revealed the second magic reserve in the standard wizard body, I've speculated that Natsu has somehow been able to open it on his own without knowing how when he ate something that wasn't fire and filled it with that element. In this case, it'd probably be filled with lightning, and now that he has more access to it, he can use it more at will.